At present, such wires are given a helical configuration by gripping opposite ends of long lengths of wire and then spinning one end whilst the other is held stationary. It has been found that present methods of providing helical configurations are unreliable and limiting in a number of important respects. The helical pitch is liable to vary along a length of wire being twisted, to an unacceptable extent. Published tolerances on such wires are as much as plus or minus 2 mm on a 40 mm pitch, creating a discrepancy of up to 10%. Variation occurs wherever there is a slight change in metal or geometric characteristics, which inevitably happens at the ends. This is because the ends have to be gripped before any twisting takes place, for a distance sufficient for the torsional forces to be taken at the ends. Such ends do not conform and have to be cut away as waste material. Another problem is that, when a long length is twisted between end grips, its overall length is progressively reduced as it is twisted and it will pull out of its end gripping device unless this device can slide in a spring loaded fashion.
The reasons why it is functionally essential for a helical finned wire to have an accurate constant helical pitch throughout its operational length is explained in detail later with reference to drawings. In essence, if it does not, the grip provided will be largely ineffective when the connecting device is driven into a relatively weak building material such as aerated concrete, because of the destructive passage of helical fins of varying pitches progressing through it. In addition, the resistance induced in driving a helix with a non-uniform pitch, into hard materials, will be greatly increased.